The measurement of pipe circumferntial expansion in the fuel line of compression ignition engines is used to infer correct injection timing. Such data may also be analysed to show the duration and relative quantity of fuel injected for a given engine speed/load quantity of fuel injected for a given engine speed/load characteristic.
The measurement is currently made by either disconnecting the pipe and inserting a "T" piece with a pressure sensor or by clamping a sensor element in a resilient mounting, on the outside of the pipe. When fuel under pressure passes through the pipe it dilates and a variation appears in the output signal delivered by the sensor elements.
The sensor elements are normally in the form of resilient strips having a central concave section providing a concave face for embracing a circumferential portion of the pipe and with a strain gauge attached to the convex face of the concave section for measuring surface deformation of the pipe.
Two such resilient strips are then mounted respectively to a pair of articulated clamping jaws with the concave section supported on its associated jaw on a resilient backing insert.
The inbuilt resiliency both in respect of the support material and the backing insert improves sensitivity and protects the strain-gauge against pressure damage when the jaws are being clamped to the pipe. It also provides means for absorbing pipe vibrations which would otherwise cause the electrical pick-up from the strain-gauge to produce parasitic signals tending to swamp the useful measuring signals.
Such sensor elements are however still liable to premature failure and the effects of pipe vibraton on signal purity is still a detrimental factor.
The former problem manifests itself primarily due to use of the sensor elements on circular pipes having radii less than the radius of the concave embracing section so that when the clamping force is applied the concave section is caused to stretch excessively thereby overloading the strain-gauge well beyond its limitations.
While the latter problem has been mitigated to some degree as explained above, difficulties in encountering certain effects of pipe vibration for instance the movement of the clamping jaws along the pipe, still remain, these residing mainly in the design of the sensor elements and the manner in which the elements are mounted to the clamping jaws to make full use of the applied clamping force in holding the elements in position on the pipe surface.